The present applicant has already suggested an imaging apparatus for acquiring a differential image between an image (emission image) taken while irradiating light by a light irradiation means and an image (non-emission image) taken without irradiating light by a light irradiation means (see JP2006-121617A). In such differential image, only the reflected light components of the light irradiated from the light irradiation means are used as pixel values. This helps reduce the influence of ambient light such as sunlight or the like.
In the imaging apparatus described above, if a light-transmitting obstacle such as glass (hereinafter just referred to as “obstacle”) exists between a subject and the imaging apparatus, an exposure amount of the subject in the emission image is decreased as the transmittance of the obstacle becomes lower. As a result, quality of the subject image taken through the obstacle may be deteriorated.
In this regard, the deterioration of the quality of the subject image can be restrained by evaluating the exposure amount of the subject image existence range in the taken image (the differential image), namely the amount of light received by an imaging element (which holds true in the following description), and adjusting the evaluated exposure amount to an appropriate value. In such an instance where a driver who drives a motor vehicle is imaged from the front side, a light-reflecting object (vehicle body) exists around a light-transmitting obstacle (vehicle windshield). Therefore, it is not easy to appropriately adjust the exposure amount of the driver (subject image) taken through the vehicle windshield.
In the event that moving target objects (subjects) or target objects differing in size are randomly imaged by the aforementioned imaging apparatus, the light can be reliably irradiated on the target objects by broadening the irradiation range of the light irradiation means and increasing the light emission amount thereof.
If the irradiation range of the light irradiation means is broadened and if the light emission amount thereof is increased, the ratio of the light not irradiated on the object grows larger depending on the situation (position or size) of the target object. Thus, electric power is unnecessarily consumed in the light irradiation means.
In the meantime, the aforementioned conventional imaging apparatus may be used in normally imaging a target object (a human or a motor vehicle) going in and out of a target region. In this case, if a constant amount of light is irradiated regardless of the existence or absence of a target object in the target region, there is posed a problem in that electric power is unnecessarily consumed in the light irradiation means and the lifespan of the light irradiation means becomes shorter.
In the aforementioned conventional imaging apparatus, if a contaminant adheres to an imaging lens or a transparent cover for protection of the imaging lens or if a scratch is generated in the imaging lens or the transparent cover, there is a likelihood that the amount of light received is decreased and the quality of the differential image is reduced. Similarly, it is likely that the amount of light received is decreased and the quality of the differential image is reduced due to the aging deterioration of a light source for irradiating light (e.g., a light emitting diode). In the event that the imaging apparatus is used for monitoring purposes, it is difficult to perceive contamination of the lens or the cover because a monitoring camera is installed in a higher position.
In the aforementioned imaging apparatus, the exposure amount of the subject in the emission image is decreased as the distance to the subject grows larger. The exposure amount is saturated if the distance to the subject is too small. It is therefore necessary to evaluate the exposure amount of the subject image existence range in the taken image (the differential image) and to adjust the evaluated exposure amount to an appropriate value.
However, if the subject is moving within the target region, the position of the subject image is changed on a frame-by-frame basis. The exposure amount cannot be appropriately evaluated if the evaluation range is kept fixed.